Projection systems may use a colour wheel to generate light of different colours from a light source. The light source typically provides white light. The colour wheel may comprise a circular substrate with surface segments of different colours. When the colour wheel is rotated with light incident upon it, its output provides light of varying colour.
A phosphor wheel is a similar device in which some or all segments of the surface of circular substrate are coated with a wavelength conversion material, such as a phosphor. The phosphor is often mixed with the glue or other transparent material for application to the substrate surface. Different phosphors may be used in different segments to provide more than one emission colour output.
Wavelength conversion materials such as phosphors receive and absorb excitation light of a first wavelength and emit light of a second, different wavelength. They may be used to generate light of specific wavelengths, where the optical power output for light sources providing that wavelength directly is limited.
Improving the efficiency of phosphor wheels is a challenge, particularly for use in projection systems. Projection systems are often desirably compact and can also have thermal requirements.
U.S. Pat. No. 7,547,114 describes a phosphor wheel in which a dichroic element is disposed between excitation light and the wavelength conversion material. The dichroic element transmits the excitation light and reflects light emitted by the wavelength conversion material, thereby improving the proportion of received light that is converted. This results in an efficiency improvement.
U.S. Pat. No. 7,651,243 also relates to a phosphor wheel and shows a recycling element, which passes light emitted by the phosphor within a range of angles, but reflects light emitted outside that angle range. This focuses the emitted light, also to increase efficiency.
These efficiency improvements are incremental. It is desirable to addressing other issues in wave conversion materials to further improve their efficiency.